A medical device that may be implanted in a patient's body is disclosed. An illustrative medical device has an elongated structure that includes a material loaded with a radiopaque material. The radiopaque material includes radiopaque particles having multiple sets of radiopaque particles. The multiple sets of radiopaque particles may include a first set of radiopaque particles having particles with particle sizes in a first particle size range and a second set of radiopaque particles having particles with particle sizes in a second particle size range. The second set of radiopaque particles may have particles configured to fit within interstitial spaces between particles of the first set of radiopaque particles.